In the range between 20kV to 30kV the inner liner tube voltage of 8kV will be switched off. This voltage is used at primary energies below 20kV as a beam booster to stabalize the "slow" electron beam and to make it more resistant against small disturbing electric or magnetic fields. Additionally the beam booster voltage is used to accelerate the returning secondary electrons onto the photosensitive layer of the detector. This means, that for primary energies above 20kV the image becomes very dark (it is still possible to detect an image, but the signal to noise ratio is quite low).
In this very high primary energy end an external SED detector is recommeded and bypasses the problem with the missing beam booster voltage.
In the range between 2kV and 0.1kV the maximum working distance from the end of the UHV-Gemini column to the sample will decrease with smaller primary energy. At a primary energy of 1kV the maximum working distance is approx. 7.8mm and with 0.5kV the maximum working distance will drop down below 4mm. Using some spacers underneath the sample can overcome this problem for pure SEM measurements (not possible for SAM).
Setting the lowest possible primary energy of 0.1kV and moving the sample to high working distances will change the electrooptical properties and remanent mangetic stray fields of the optical lens-system will force the very slow electons on trajectories which will cover the whole hemisphere around the column.
Under certain conditions this hemispheric scanning electron beam can be focussed with infinity depth of field, which produces an image which looks similar to camera images with a fisheye lens.
EHT=0.1kV, Internal SED
|
EHT=0.1kV, External SED
|
When the internal SED is used, only the parts of the hemisphere near to the UHV-Gemini will produce enough secondary electrons deflecting back into the Gemini to produce a sufficiant signal at the detector.
In case an external SED is available and will be used as the active detector, more electrons even from points much more further out in the chamber can be detected.
A combination of internal and external SED will give the best result for a "fisheye" view of the sample and its surrounding.
EHT=0.1kV, Mixture of Inernal and External SED
|
A rough guideline how to set parameters for obtaining high quality "fisheye" images with the UHV-Gemini can be found below:
- Set EHT to 0.1kV
- Choose a beam current of 3.0nA
- Change working distance (focus setting) to 2.5mm (e.g. double-click on WD at data-zone and type in "2.5")
- Always zoom out as much as possible
- Find best–focussed image by changing Aperture / Gun alignment (try to keep fisheye-image near the center of the screen.
- For taking image make data-zone disappear (toggle Data-zone with "CTRL-D"), change to higher resolution (e.g. 2048x1536 or 3072x2304) and use Scan Speed 10
- Best results can be achieved with Internal / External SED mixed signal
In case the image can not be focussed properly try to use different parameters, like beam current 2.5nA or 2.0nA and change the focus setting and gun / aperture alignment. The main parameter for focussing properties is the condenser current, which can only be changed in big steps via the beam current selection. The objective current (focus) will also change the focussing properties of the fisheye image, but will lead to an increase of the fisheye-diameter.
A good compromise between well focussed image and fisheye-view fitting onto the screen should be possible to find
Unfolding the Fisheye
The next step for receiving a nice panorama view is unfolding the fisheye image properly. This procedure can be done with serveral freeware or commercial software which are capable of distortion correction in photography.
One easy solution shown here is a special java-plugin for the excellent freeware program "ImageJ" http://rsb.info.nih.gov/ij/.
ImageJ Main Window
|
The newest version of the main program can be downloaded and installed from the download page of ImageJ.
The "Java-bundled" version should be used.
The java-plugin "Fisheye_To_Panorama" (files 'Fisheye_To_Panorama.java" and 'Fisheye_To_Panorama.class") should be copied to the "plugins" folder of the new installed program "ImageJ" before starting the main program.
ImageJ Circular ROI
|
First of all the image containing the fisheye-view must be opened with the program ImageJ. For using the plugin the image must be of the type 8-bit grayscale, 16-bit grayscale or RGB.
Using RGB images might lead to false colors in the result-image, thus I would recommend to use 8-bit or 16-bit grayscale images (the type of image can easily be converted with ImageJ with "Image -> Type -> 8-bit"(/16-bit/RGB/...etc.).
To start the plugin select the image to convert (just keep this window the last activated before choosing the plugin), and
use an ellipse type ROI (Region of Interest) to mark the position of the round-shaped fisheye. The ROI must be circular and should contain the whole fisheye.
To receive a circular ROI press down the SHIFT key when you drag an elliptical ROI around the desired area.
After adjustement of the circular ROI activate the plugin by "Pugins -> Fisheye to Panorama". A setup window will pop up to give the opportunity for setting some important parameters in advance.
Parameters Setup Window
|
These parameters are:
- Use linear Interpolation:
If this option is checked, each pixel of the new created panorama image will be calculated from a mean value of a circular shaped area with diameter "d" around the converted pixel of the original image.
The diameter "d" can be chosen as "Interpolation Diameter".
- Use Gauss and Sharpen after Calculation:
Using this option will first calculate the new panorama image and after finishing it will perform a Gauss blur procedure with given FWHM as the "Gauss Blur Radius"-parameter. After the Gauss blur function a "Sharpen" function will be applied. Frayed out edges will become more smooth with this combination of filters.
- Auto Save Panorama and FSV-ASCII-File:
This auto-save function will open a save-file dialog after calculation of the panorama image. A folder and name of the new panorama image can be chosen. Additionally a setup-file (ASCII) for the panorama viewer program (FSP-Viewer) will be created (see info about this program later).
- Auto Save Panorama and PTViewer-HTML-File:
A save-file dialog will open after panorama calculation and folder and name of the new panorama image can be chosen. Additionally a simple HTML-file will be created which contains the setup and call of the java-applet "PTViewer". "PTViewer" is a freeware panorama viewer as java-applet for integration into html-pages.
The new panorama image has an empty area at the top of the image. This area is left clear as dummy for the full sphere projection. PTViewer needs full projection panorama images.
- Y Scaling Factor:
The size of the created panorama image will be perimeter of the circular ROI in width (x-direction) and radius of the ROI in height (y-direction). The scaling factor will stretch the Y-axis to adjust and improve the x to y ratio. This value might be different for different type of panorama-viewers.
- FSP-Viewer Parameters:
Here some special parameters for the FSP-Viewer can be set, which will be included in the ASCII FSP-file in case "Auto Save Panorama and FSP-File" is selected. The default values of these Presets are adjusted to the needed properties of the UHV-Gemini fisheye view at 0.1kV.
Confirming with "OK" will start the calculation of the panorama image.
Calculated Panorama Image for FSP-Viewer
|
Using FSP-Viewer (for high resolution images):
In case "Auto Save Panorama and FSV-ASCII-File" was selected, the matching FSV-ASCII-file will include the name of this file (you do have to change this name in the ASCII-file, in case you will copy the files to a different folder!). The file also contains the preselected parameters.
ASCII Setup File for FSP-Viewer
|
To watch the panorama image a program called "FSP-Viewer" can be used. FSPViewer is a free viewer for spherical (equirectangular) panoramic images. It can be downloaded at: http://www.fsoft.it/panorama/FSPViewer.htm
The program can display the calculated panorama as a 360° projection in a separate window (this window can also be in full-frame mode) of very high quality.
Download the program and unzip it to a folder of your choice. The program does not need to be installed.
To watch the created panorama image, double click onto the ASCII FSV-File in the windows explorer and choose this file extension to be permanently linked with the FSP-Viewer.
A short describtion of the FSP-Viewer can be found in the folder you unzipped the downloaded program to. The help file is called "FSPViewer.htm".
Using PTViewer (for low resolution images and browser java-applet):
Calculated Panorama Image for PTViewer
|
If the panorama was saved with the selected option " Auto Save Panorama and PTViewer-HTML-File" an additional HTML file is created which has the same name like the panorama-images, but with ending ".html". Here the basic parameters for the java-applet "PTViewer" are included in a very simple HTML-file.
The Java-applet "PTViewer" can be downloaded from http://webuser.fh-furtwangen.de/~dersch/.
Download the Java Viewer for HDR-Panoramas (PTViewer 3.1.2, Java Applet) and place the "ptviewer.jar"-file in the same folder like the created panorama-image and HTML-file.
Opening the HTML-file and a clicking with the left mouse button onto the applet-area should activate the plugin (assuming Java is activated on your PC).
Note, that large panorama-images might have problems to load into the browser. I would recommend to use panorama image sizes of 3000 pixel in x as maximum (maybe resizing with a graphics program, e.g. with ImageJ, might be necessarry).
Press left mouse button and move mouse to look around; Press SHIFT (CTRL) and left mouse button to zoom in (out)
